Nickel compounds

ABSTRACT

Nickel compounds of the formula (R 1  COO) (R 2  COO) Ni, where R 1  is hydrocarbyl of at least 5 carbon atoms and R 2  is haloalkyl of 1-3 carbon atoms, are manufactured from the corresponding acids R 1  COOH and R 2  COOH or salts thereof. The resultant nickel compounds are soluble in hydrocarbons and can be used in admixture with organoaluminum compounds as constituents of catalytic compositions; olefins can be oligomerized, for example, dimerized and trimerized, therewith.

BACKGROUND OF THE INVENTION

The present invention relates to new bivalent nickel compounds, theirmanufacture and use as components of catalytic compositions foroligomerizing olefins.

Bivalent nickel inorganic salts are well-known. They are usually solublein an aqueous medium and have little solubility, if any, in ahydrocarbon medium. Conversely, bivalent nickel salts of carboxylicacids of the formula (RCOO)₂ Ni, where R is a substituted orunsubstituted hydrocarbyl radical, are usually soluble, sometimes inlarge amounts, in hydrocarbon media, provided the radical R has asufficient number of carbon atoms. This property is often desired whenthe nickel salts must be used as homogeneous catalysts, either alone orassociated with Lewis acids, such as alkylaluminum compounds. It is truethat the use of insoluble nickel salts as catalysts has several knowndisadvantages, particularly a weaker activity as compared with solublesalts and difficulties of use, particularly in continuous industrialoperations where precise and particularly low proportions of catalystmust be used. This explains why it has already been proposed to preparesoluble oligomerization catalysts, such as catalysts for dimerizing orco-dimerizing mono-olefins, by reacting nickel carboxylates withhydrocarbyl aluminum halides.

The use of these catalysts has, however, the disadvantage that theactivity observed in continuous operations is often lower than in batchoperations, and even tends to decrease in the course of time. Nosatisfactory explanation of this phenomenon has yet been found.

SUMMARY

The object of the present invention is precisely to provide new mixednickel compounds, soluble in hydrocarbon media, which, when used withhydrocarbyl aluminum halides in oligomerization operations, haveincreased catalytic activity and stability, particularly as concernscontinuous operations. This activity is even higher than that obtainedby joint use of a nickel carboxylate, a halogenoacetic acid and ahydrocarbyl aluminum halide, as disclosed in U.S. application Ser. No.102,488. These new compounds are also useful, when associated with alkylaluminum compounds, as catalysts for diolefin polymerization.

These new mixed nickel compounds are of the general formula (R₁ COO) (R₂COO)Ni, where R₁ is a hydrocarbyl radical, for example an alkyl,cycloalkyl, alkenyl, aryl, aralkyl or alkaryl radical, containing atleast 5 carbon atoms, preferably a hydrocarbyl radical of 5-20 carbonatoms, which radical may be substituted with, for example, hydroxygroups, and R₂ is a halogenoalkyl radical containing from 1 to 3 carbonatoms, of the formula C_(m) H_(p) X_(q), where m is 1, 2 or 3, p is zeroor an integer and q is an integer, provided that p+q=2m+1. R₂ ispreferably a halogenomethyl radical CX_(n) H_(3-n) where X is fluorine,chlorine, bromine or iodine and n is an integer from 1 to 3.

The high activity of the catalytic compositions obtained from the abovenickel compounds and hydrocarbyl aluminum halides is the more unexpectedas the catalytic compositions obtained from nickel bis(halogenoacetates) and hydrocarbyl aluminum halides have themselves arelatively poor activity, probably due to the insolubility of thesenickel compounds in hydrocarbon media. The new nickel compoundsaccording to the invention have conversely a substantial solubility inhydrocarbon media, i.e. a solubility of at least 0.1 g per liter underthe conditions of use.

The mixed compounds (R₁ COO) (R₂ COO)Ni may be manufactured by reactingthe mixture of the two acids R₁ COOH and R₂ COOH in substantiallyequimolecular quantities with nickel metal or a nickel oxide, hydroxideor carbonate. They can also be prepared in aqueous medium by contactingwater-soluble alkali metal salts (including ammonium salts) of the twoacids R₁ COOH and R₂ COOH in proportions which are preferablysubstantially equimolecular, with a bivalent nickel salt soluble inwater. Examples of useful salts are nickel chloride, nickel bromide,nickel iodide, nickel sulfate and nickel nitrate and their hydrates. Thealkali metal salts may be formed in situ by using organic acids andoperating in the presence of a base providing the alkali metal.

The reaction is preferably effected in a liquid two phase mediumcomprising an aqueous phase and a substantially immiscible organicphase, consisting of aliphatic, cycloaliphatic or aromatic hydrocarbonsor mixtures thereof, or of halogenated hydrocarbons, for example,chlorinated, fluorinated or brominated hydrocarbons. Use may be made of,for example, pentane, heptane, petroleum ether, naphtha, cyclohexane,toluene, xylenes, ethylbenzene, methylene chloride or chloroform. Theaqueous phase and the alkali metal salt formed by double decompositionare separated thereafter.

According to another operating procedure, the mixture of the alkalimetal salts of the two acids R₁ COOH and R₂ COOH may be prepared in alower alcohol having, for example, 1 to 4 carbon atoms, for example,methanol or ethanol, and the nickel salt is added thereafter. Afterreaction, the alcohol is removed at least in a major part, bydistillation and is replaced with a hydrocarbon or a halogenatedhydrocarbon as above described. The inorganic salts can then beeliminated by filtration.

According to a modified embodiment, the mixture of the alkali metalsalts of the two acids R₁ COOH and R₂ COOH may be prepared in a mixedmedium comprising an alcohol, for example methanol or ethanol, and ahydrocarbon or a halogenated hydrocarbon as above described. Thecomposition of the alcohol-hydrocarbon mixture may be selected at will;for example, the mixture may have the same composition as the azeotrope,if any, formed by the two solvents. The alcohol or solvent mixture canbe removed thereafter.

Another operating procedure, which also leads to the mixed compounds (R₁COO) (R₂ COO)Ni, consists of separately preparing, according to knownmethods, the two compounds (R₁ COO)₂ Ni and (R₂ COO)₂ Ni and thenreacting these two compounds in substantially equimolecular proportionsin a liquid medium comprising a polar solvent, for example, water or analcohol, and a non-polaar solvent, for example a hydrocarbon or ahalogenated hydrocarbon, and subsequently eliminating the polar solventby distillation.

When using free acids as starting compounds, the base providing thealkali metal is preferably used in substantially stoichiometric amountwith respect to the two acids in the form of, for example, sodium orpotassium hydroxide, or the corresponding carbonates orhydrogenocarbonates, or ammonia.

It may however be advantageous, in order to facilitate the isolation ofthe nickel salt, to operate with a slight insufficiency, for example of5-30%, with respect to the stoichiometry, of one of the reactants, forexample an insufficient amount of the base.

The acids or their salts may be used in stoichiometric proportions withrespect to the nickel salt, but different proportions can also be used.Thus an excess of nickel salt, for example a molar excess of 5 to 20%,is favorable to the recovery of a pure product.

The order of supply of the reactants is not critical. However it ispreferred to admit first the two acids R₁ COOH and R₂ COOH with the basein a polar medium, so as to obtain the salts, and then introduce thenickel salt and the hydrocarbon phase.

The proportion of water or alcohol to be used is not critical; however,according to the invention, the best results are obtained with theminimum amount necessary to dissolve the reactants at the reactiontemperature. The amount of hydrocarbon phase must be sufficient todissolve the mixed salt. It then depends on the nature of the reactants;a preferred amount is selected between 2 and 100 times the amount ofwater or alcohol.

The reaction temperature depends on the reaction facilities; it ishowever preferred to heat the reaction mixture at reflux underatmospheric pressure, or at a higher or lower pressure, up to stablecoloration of the organic phase. This operation may take from 5 minutesto 5 hours, depending on the reflux temperature.

The polar phase, containing the inorganic salts, may be withdrawn bydecantation and discharged; it has, however, been observed, and this isanother particular feature of the invention, that particularly highreaction velocities and yields, as concerns the mixed nickel compounds,are obtained by heteroazeotropic distillation of the polar solvent. Thisdistillation is preferably continued up to complete removal of the polarphase. In that case, the resultant inorganic salts are separated bydecantation or filtration. The hydrocarbon or halogenated hydrocarbonsolvent may be evaporated or distilled and the nickel compound may beisolated as a generally amorphous green solid; for specific uses,however, it can be used as a solution.

Another object of the present invention is to use these mixed bivalentnickel salts as components of a new catalyst composition useful foroligomerizing, and particularly dimerizing and trimerizing mono-olefins.This new composition concerns more precisely the combinations obtainedby contacting, in any order, at least one mixed nickel salt with atleast one hydrocarbyl aluminum halide.

The following compounds may be used, alone or as mixtures, the listbeing however not limitative; nickel 2-ethylbutyrate trifluoroacetate,nickel 2-ethylbutyrate trichloroacetate, nickel 3,3-dimethylbutyratetrifluoroacetate, nickel 3,3-dimethylbutyrate trichloroacetate, nickel4-methylvalerate trifluoroacetate, nickel hexanoate trichloroacetate,nickel heptanoate trifluoroacetate, nickel heptanoate trichloroacetate,nickel heptanoate tribromoacetate, nickel heptanoate triiodoacetate,nickel 2-ethylhexanoate trifluoroacetate, nickel 2-ethylhexanoatemonofluoroacetate, nickel 2-ethylhexanoate trichloroacetate, nickel2-ethylhexanoate dichloroacetate, nickel 2-ethylhexanoatemonochloroacetate, nickel 2-ethylhexanoate tribromoacetate, nickel2-ethylhexanoate triiodoacetate, nickel octoate trifluoroacetate, nickeloctoate trichloroacetate, nickel decanoate trifluoroacetate, nickeldecanoate trichloroacetate, nickel myristate trifluoroacetate, nickelpalmitate trifluoroacetate, nickel dodecylbenzoate trifluoroacetate,nickel diisopropylsalicylate trichloroacetate, nickel myristatepentafluoropropionate and nickel 2-ethylhexanoate heptafluorobutyrate.

Preferred hydrocarbylaluminum halides conform to with the generalformula Al₂ R_(x) Y_(y) where R is a hydrocarbon group containing up to12 carbon atoms, or more, such as alkyl, aryl, aralkyl, alkaryl,cycloalkyl; Y represents halogen: fluorine, chlorine, bromine or iodine;x and y have each a value of 2,3 or 4 with x+y=6. Examples of suchcompounds are ethyl aluminum sesquichloride, dichloroethylaluminum,dichloroisobutylaluminum, chlorodiethylaluminum or their mixtures.Examples of catalyst compositions consist of any one of the mixed nickelcompounds of the above list and any one of the aluminum compoundsmentioned above.

The invention has also for an object, a process for oligomerizingmono-olefins in the presence of the above catalytic compositions at atemperature of -20° C. to +80° C. at pressure conditions such that thereactants are maintained, at least in a major part, in the liquid orcondensed phase.

Mono-olefins which can be dimerized or oligomerized are, for example,ethylene, propylene, n-butenes, n-pentenes, either pure or as mixturessuch as recovered from synthesis processes, for example, catalyticcracking or steam-cracking. They can be co-oligomerized either togetheror with isobutene, for example, ethylene with propylene and n-butenes,propylene with n-butenes or n-butenes with iso-butene.

The concentration, expressed as nickel, of the catalytic composition inthe liquid phase of the oligomerization reaction, is normally comprisedbetween 5 and 500 parts per million by weight. The molar ratio of thehydrocarbyl aluminum halide to the nickel compound is normally comprisedbetween 1:1 and 50:1, more advantageously between 2:1 and 20:1.

The process may be operated with separate charges but offers a maximumof advantages when operated continuously. In that case, a reactor withone or more serially arranged reaction stages may be used, the olefiniccharge and/or the catalyst constituents being continuously introducedeither into the first stage or in the first and any other stage. Thesecond and n^(th) stage may also be fed only with additional amounts ofone constituent of the catalytic mixture.

At the outlet of the reactor, the catalyst may be deactivated in a knownmanner, for example, with ammonia and/or an aqueous sodium hydroxidesolution and/or an aqueous sulfuric acid solution. The unconvertedolefins and the alkanes, if any, are separated thereafter from theoligomers by distillation.

The following examples illustrate the invention but constitute nolimitation thereof. The characteristic portion of the infra-redabsorption spectrum of the compounds obtained in examples 1 to 5 isshown in FIGS. 1 to 5 (Nujol, KBr). A is the wave length in microns andB the wave number in cm⁻¹.

EXAMPLE 1

14.4 g of 2-ethyl hexanoic acid (0.1 mole), 200 cm³ of heptane, 11.4 gof trifluoroacetic acid (0.1 mole), 10.6 g of sodium carbonate, 23.7 gof nickel chloride hexahydrate (0.1 mole) and 25 cm³ of water aresuccessively introduced into a 500 cm³ glass reaction vessel providedwith a magnetic stirring rod and a device for heteroazeotropicdistillation. The contents are heated to reflux and water is eliminatedby azeotropic distillation. An insoluble precipitate containing NaCl isfiltered and washed with heptane; the organic phase is evaporated undervacuum. There remains a solid of intense green color, which isidentified by elemental analysis and infra-red spectrometry as nickel2-ethyl hexanoate trifluoroacetate. Elemental analysis: calculated forC₁₀ H₁₅ O₄ F₃ Ni: C=38.1; H=4.8; Ni=18.7; found (%): C=38.7; H=5.6;Ni=16.9. Infra-red spectrum: characteristic bands at 1670 cm⁻¹ (CF₃COO), 1575 and 1410 cm⁻¹ (C₇ H₁₅ COO), 1205 and 1150 cm⁻¹ (CF₃). 29.5 gof the mixed salt have been obtained, corresponding to a yield of 93.1%.

It has been proved that the above salt is a mixed compound and not amixture of nickel trifluoroacetate with nickel 2-ethyl hexanoate: nickeltrifluoroacetate is insoluble in hydrocarbons; if formed, it would havebeen eliminated in the solid state and the yield of hydrocarbon-solublecompound would have been far lower.

EXAMPLE 2

The operation is conducted as in example 1, while successivelyintroducing: 80 cm³ of heptane, 5 cm³ or water, 0.91 g oftrifluoroacetic acid (8×10⁻³ mole), 1.04 g of heptanoic acid (8×10⁻³mole), 0.64 g of sodium hydroxide (1.6×10⁻² mole) and finally 2.81 g ofnickel sulfate heptahydrate (10⁻² mole). A green solid is obtained(weight: 2.23 g; yield: 88.8%) identified as being nickel heptanoatetrifluoroacetate. Elemental analysis: calculated for C₉ H₁₃ O₄ F₃ Ni:C=35.9; H=4.3; F=18.9; Ni=19.6; found (%) C=35.6; H=4.5; F=18.7;Ni=19.1. Infra-red spectrum: characteristic bands at 1675 cm⁻¹ (CF₃COO), 1570 cm⁻¹ (C₆ H₁₃ COO), 1205 and 1150 cm⁻¹ (CF₃).

EXAMPLE 3

The operation is conducted as in example 1, while reacting: 80 cm³ ofheptane, 5 cm³ of water, 0.91 g of trifluoroacetic acid (8×10⁻³ mole),1.71 g of myristic acid (8×10⁻³ mole), 0.64 g of sodium hydroxide(1.6×10⁻² mole) and 2.81 g of nickel sulfate heptahydrate (10⁻² mole). Agreen solid (2.80 g) is obtained and identified as being nickelmyristate trifluoroacetate (yield: 90.9%). Elemental analysis:calculated for C₁₅ H₂₅ O₄ F₃ Ni: C=46.8; H=6.5; F=14.8; Ni=15.3; found(%) C=47.7; H=7.1; F=14.4; Ni=14.8. Infra-red spectrum: characteristicbands at 1675 cm⁻¹ (CF₃ COO), 1580 and 1410 cm⁻¹ (C₁₂ H₂₅ COO), 1205 and1150 cm⁻¹ CF₃).

EXAMPLE 4

The operation is conducted as in example 1, while reacting: 100 cm³ ofheptane, 25 cm³ of water, 5.75 g of 2-ethyl hexanoic acid (4×10⁻² mole),6.55 g of trichloroacetic acid (4×10⁻² mole), 3.20 g of sodium hydroxide(8×10⁻² mole) and 14.05 g of nickel sulfate heptahydrate (5×10⁻² mole).A green solid is obtained (10.8 g), which is identified as being nickel2-ethyl hexanote trichloroacetate (yield: 74.2%). Elemental analysis:calculated for C₁₀ H₁₅ O₄ Cl₃ Ni: C=32.9; H=4.1; Ni=16.2; found (%)C=37.5; H=5.7; Ni=15.2. Infra-red spectrum: characteristic bands at 1655cm⁻¹ (CCl₃ COO), 1570 and 1410 cm⁻¹ (C₇ H₁₅ COO), 845 and 835 cm⁻¹(CCl₃).

EXAMPLE 5

The process is conducted as in example 1, while reacting: 80 cm³ ofheptane, 5 cm³ of water, 0.91 g of tricluoroacetic acid (8×10⁻¹ mole),0.93 g of 4-methyl valeric acid (8×10⁻³ mole), 0.64 g of sodiumhydroxide (1.6×10⁻² mole) and then 2.81 g of nickel sulfate heptahydrate(10⁻² mole). A green solid is obtained (0.6 g) and identified as beingnickel 4-methyl valerate trifluoroacetate (yield: 26%). Elementalanalysis: calculated for C₈ H₁₁ O₄ F₃ Ni: C=33.4; H=3.8; Ni=20.5; found(%) C=33.3; H=3.9; Ni=20.1. Infrared spectrum: characteristic bands at1675 cm⁻¹ (CF₃ COO), 1570 and 1410 cm⁻¹ (C₅ H₁₁ COO), 1200 and 1145 cm⁻¹(CF₃).

EXAMPLE 6

An oligomerization reactor consists of two serially arranged reactionstages, each consisting of a double-jacket cylindrical steel reactor ofa useful volume of 0.25 liter, whose temperature is controlled bycirculating water.

The first stage reactor is continuously fed with 98 g/h of a C₄ cutwhose composition is as follows:

propane: 1.1% b.w.

isobutane: 6.7

n-butane: 23.0

1-butene: 5.2

trans 2-butene: 46.4

cis 2-butene: 17.6

The reactor is also fed with 0.031 g/h of nickel 2-ethyl hexanoatetrifluoroacetate prepared as in example 1, as a solution in heptane, and0.194 g/h of dichloroethylaluminum as a solution in heptane. A pressureof 5 bars is maintained in the reactors by continuously discharging thereaction product, and the temperature is maintained at 42° C. by meansof circulating water.

After 4 hours of run, a steady state run is obtained, corresponding to a66% conversion of the butenes to oligomers in the first stage and 75% atthe outlet of the second stage. The oligomers comprise 85% dimers(n-octenes, methylheptenes and dimethylhexenes), 12% trimers and 3%tetramers.

The activity of the catalyst system, expressed as the velocity constantk (mol⁻¹.l.h⁻¹) defined by the relation V=kC², where V is the reactionvelocity (mol.l⁻¹.h⁻¹) and C the butenes stationary concentration(mol.l⁻¹) in the stages, is:

k (first stage): 0.423 mol⁻¹.l.h⁻¹

k (second stage): 0.106 mol⁻¹.l.h⁻¹

EXAMPLE 7

The apparatus is the same as in example 6 and it is continuously fedwith 83 g/h of the same C₄ cut, 0.036 g/h of nickel 2-ethyl hexanoatetrichloroacetate prepared as in example 4 in the form of a solution inheptane, and 0.194 g/h of dichloroethylaluminum as a solution inheptane.

After 4 hours of run, a steady state operating rate is attained,corresponding to a butenes conversion of 64% in the first stage and 77%at the outlet of the second stage.

The activity of the catalyst system, expressed as in example 6, is:

k (first stage): 0.307 mol⁻¹.l.h⁻¹

k (second stage): 0.152 mol⁻¹.l.h⁻¹

EXAMPLE 8

This example is given by way of comparison for showing the advantages ofthe invention.

The apparatus is the same as in example 6. It is continuously fed with82 g/h of the same C₄ cut, 0.052 g/h of a nickel carboxylate having a11% b.w. nickel content, admixed with 0.011 g/h of trifluoroacetic acidin heptane, and 0.194 g/h of dichloroethylaluminum as a solution inheptane.

The amount of nickel, expressed in g/h of Ni metal, is thus identical tothat used in example 6, which permits a direct comparison of the resultsby means of the constant k.

The steady state operating rate attained after 4 hours of runcorresponds to a 57% conversion in the first stage and 68% at the outletof the second stage. The activity of the catalyst system is thussubstantially lower than in example 6:

k (first stage): 0.193 mol⁻¹.l.h⁻¹

k (second stage): 0.067 mol⁻¹.l.h⁻¹

EXAMPLE 9

14.4 g of 2-ethyl hexanoic acid (0.1 mole), 100 cm³ of methanol, 11.4 gof trifluoroacetic acid (0.1 mole) and 8.0 g of sodium hydroxide (0.2mole) are successively fed into a 500 cm³ glass reaction vessel providedwith a magnetic stirring rod and a reflux device. The mixture is heatedto reflux and 35.1 g (0.125 mole) of nickel sulfate heptahydrate isadded. The mixture is heated again at reflux. Methanol is evaporated bymeans of a rotary evaporator and the residue is taken up in toluene.After filtration of the insoluble inorganic salts, toluene is distilledoff and nickel 2-ethyl hexanoate trifluoroacetate is obtained. Itsanalysis corresponds to the formula C₁₀ H₁₅ O₄ F₃ Ni and it has the samecharacteristic bands as the product of example 1.

EXAMPLE 10

An oligomerization reactor of 35 liter useful volume is continuously fedwith 5 kg/h of a C₃ cut having the following composition:

propane: 20% b.w.

propylene: 80% b.w.

0.273 g/h of nickel 2-ethyl hexanoate trifluoroacetate as a solution inisooctane and 1.65 g/h dichloroethylaluminum as a solution in isooctaneare added. A pressure of 15 bars is maintained by withdrawal of reactionproduct and a temperature of 42° C. by water circulation. The conversionof propylene to oligomers under steady state conditions amounts to 90%and the resultant oligomers comprise 85% of dimers (n-hexenes,methylpentenes and dimethylbutenes), 12% of trimers and 3% of tetramers.

EXAMPLE 11

25 g of NiO, 46 cm³ of trifluoroacetic acid and 90 cm³ of distilledwater are introduced into a 1-liter glass reaction flask provided with amagnetic stirring rod and a distillation device. The mixture is heatedat reflux for 2 hours; then the excess of NiO is filtered. An aqueoussolution of nickel trifluoroacetate Ni(CF₃ COO)₂ is thus obtained inquantitative yield. 135 g of nickel 2-ethyl hexanoate (13% b.w. nickelcontent) of technical grade, containing 10% of free 2-ethyl hexanoicacid, dissolved in 500 cm³ heptane, are added. After heating at reflux,water is removed by azeotropic distillation. After complete waterremoval, heptane is separated by evaporation under vacuum. Nickel2-ethyl hexanoate trifluoroacetate is thus obtained with a yield of 98%.Elemental analysis: calculated for C₁₀ H₁₅ O₄ F₃ Ni: C=38.1; H=4.8;Ni=18.7; found: C=38.4; H=5.4; Ni=17.8. The infra-red spectrum shows theexpected characteristic bands identical to those of example 1.

The above nickel compound has been used to prepare an oligomerizationcatalyst and to oligomerize a C₄ cut under the same conditions as inexample 6.

The conversion was 66% in the first stage and 75% at the outlet of thesecond stage and the oligomers analyzed substantially as in example 6.

To summarize, the mixed salts of the invention may be prepared byreacting a source of R₁ COO--ion in solution with a source of R₂COO--ion in solution and a source of nickel ion (which may be commonwith the two above sources) in solution. The solvent used for thedissolution may be common to all the sources or distinct solvents may beused. A polar solvent, preferably water and/or an alcohol, is commonlyused. Simultaneously, or subsequently, a hydrocarbon solvent or ahalogenated hydrocarbon solvent is added and the polar solvent isremoved, preferably by distillation. The mixed salt is obtained as asolution in the hydrocarbon or the halogenated hydrocarbon.

What is claimed is:
 1. A new nickel compound of the general formula (R₁COO) (R₂ COO)Ni, where R₁ is a hydroxy substituted or unsubstitutedhydrocarbyl radical containing 5-20 carbon atoms and R₂ is a haloalkylgroup comprising 1 to 3 carbon atoms.
 2. A new nickel compound accordingto claim 1, where R₁ is an alkyl radical and R₂ is a CF₃ trifluoromethylor a CCl₃ trichloromethyl group.
 3. A nickel compound according to claim1, wherein R₁ is alkyl and R₂ is of the formula C_(m) H_(p) X_(q),wherein x is halogen m is 1, 2 or 3, p is zero or an integer and q is aninteger, provided that p+q=2m+1.
 4. A compound according to claim 1wherein R₁ is alkyl and R₂ is a halogenomethyl radical of the formulaCX_(n) H_(3-n) where X is fluorine, chlorine, bromine or iodine and n isan integer from 1 to
 3. 5. A nickel compound according to claim 1, being2-ethyl hexanoate trifluoroacetate.
 6. A nickel compound according toclaim 1, being nickel heptanoate trifluoroacetate.
 7. A nickel compoundaccording to claim 1, being nickel myristate trifluoroacetate.
 8. Anickel compound according to claim 1, being nickel 2-ethyl hexanoatetrichloroacetate.
 9. A nickel compound according to claim 1, being4-methyl valerate trifluoroacetate.